There have been developed electroluminescent devices using organic thin films (organic electroluminescent devices). Materials for organic electroluminescent devices can be roughly classified into low molecular weight materials and high molecular weight materials.
There have been developed organic electroluminescent devices using low molecular weight materials. Examples of such devices include an organic electroluminescent device having a hole transport layer formed from an aromatic diamine, and a light emitting layer formed from aluminum 8-hydroxyquinoline complex; and an organic electroluminescent device using aluminum 8-hydroxyquinoline complex as a host material, doped with a fluorescent dye for laser, such as coumarin. Low molecular weight materials such as the following platinum complex and iridium complex are also used as materials for light emitting layers.

There have also been developed organic electroluminescent devices using high molecular weight materials such as poly(p-phenylenevinylene)s, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]s, and poly(3-alkylthiophene)s; and devices using high molecular weight materials such as polyvinylcarbazoles, in combination with low molecular weight luminescent materials and electron transfer materials. Most of such devices using high molecular weight materials are manufactured by wet coating process such as spin coating or an ink jet process, in consideration of properties of the materials.
Focusing attention on processes for forming thin films, films of most low molecular weight materials have been formed by vacuum deposition, and films of most high molecular weight materials have been formed by wet coating process. The vacuum deposition is advantageous typically in that a film with good quality can be uniformly formed on a substrate, that a multilayer film can be easily formed to yield a device having excellent properties easily, and that contamination of impurities derived from the manufacturing process is very little. Accordingly, most of organic electroluminescent devices currently used in practice are formed by vacuum deposition using low molecular weight materials.
In contrast, the wet coating process is advantageous typically in that no vacuum process is required, that a film with a larger area can be easily obtained, and that one layer (coating composition) can contain plural materials having different functions. The wet coating process, however, has following problems, and most devices formed by wet coating process are not developed to a practical level, except for those using some high molecular weight materials.
It is difficult to control the degrees of polymerization and the molecular weight distributions of high molecular weight materials (polymerized organic compounds).
When devices are operated continuously, terminal residues cause deterioration of the devices.
It is difficult to purify high molecular weight materials highly, and the resulting materials may contain impurities.
As an attempt to solve these problems, following Patent Document 1 and Patent Document 2 each disclose the use of low molecular weight materials (unpolymerized organic compounds) each containing a fluorescent substance, a hole transport material, and an electron transport material, instead of high molecular weight materials (polymerized organic compounds). This attempt is intended to reduce the drive voltage by allowing the hole transport material and the electron transport material to transport holes and electrons injected from an anode and a cathode, respectively. The resulting devices, however, operate at high drive voltages and have insufficient luminous efficiencies, because holes and electrons are not sufficiently injected from the anode and cathode, respectively. Oxadiazole derivatives used as the electron transport material are insufficient in drive stability (operation stability) and thereby insufficient in operating life. In addition, it is difficult to adopt a phosphorescent material or a blue-emitting material as a luminescent material, because the resulting luminescent material has a large energy gap.
Patent Document 1: Japanese Patent No. 3069139
Patent Document 2: Japanese Unexamined Patent Application Publication No. 11-273859